Platform: do at least something in the test draw event.

To indicate the event handler was triggered.

src/Magnum/Platform/Test/GlfwApplicationTest.cpp

@@ -54,7 +54,10 @@ struct GlfwApplicationTest: Platform::Application {
         event.setAccepted(); /* Comment-out to test app exit suppression */
     }
 
-    void drawEvent() override {}
+    void drawEvent() override {
+        Debug{} << "draw event";
+        swapBuffers();
+    }
 
     void keyPressEvent(KeyEvent& event) override {
         #if GLFW_VERSION_MAJOR*100 + GLFW_VERSION_MINOR >= 302

src/Magnum/Platform/Test/GlxApplicationTest.cpp

@@ -29,7 +29,11 @@ namespace Magnum { namespace Platform { namespace Test { namespace {
 
 struct GlxApplicationTest: Platform::Application {
     explicit GlxApplicationTest(const Arguments& arguments): Platform::Application{arguments} {}
-    void drawEvent() override {}
+
+    void drawEvent() override {
+        Debug{} << "draw event";
+        swapBuffers();
+    }
 };
 
 }}}}

src/Magnum/Platform/Test/Sdl2ApplicationTest.cpp

@@ -57,8 +57,6 @@ struct Sdl2ApplicationTest: Platform::Application {
         event.setAccepted(); /* Comment-out to test app exit suppression */
     }
 
-    void drawEvent() override {}
-
     /* For testing HiDPI resize events */
     void viewportEvent(ViewportEvent& event) override {
         Debug{} << "viewport event" << event.windowSize()
@@ -68,6 +66,11 @@ struct Sdl2ApplicationTest: Platform::Application {
             << event.dpiScaling();
     }
 
+    void drawEvent() override {
+        Debug{} << "draw event";
+        swapBuffers();
+    }
+
     /* For testing event coordinates */
     void mousePressEvent(MouseEvent& event) override {
         Debug{} << "mouse press event:" << event.position() << Int(event.button());

src/Magnum/Platform/Test/XEglApplicationTest.cpp

@@ -29,7 +29,11 @@ namespace Magnum { namespace Platform { namespace Test { namespace {
 
 struct XEglApplicationTest: Platform::Application {
     explicit XEglApplicationTest(const Arguments& arguments): Platform::Application{arguments} {}
-    void drawEvent() override {}
+
+    void drawEvent() override {
+        Debug{} << "draw event";
+        swapBuffers();
+    }
 };
 
 }}}}